During the past year we have obtained over 60 isolates of HIV-1 from over 40 patients, many of whom have been on prolonged therapy with AZT (3'- azido, 3'-deoxythymidine, zidovudine) for AIDS or advanced AIDS-related complex. We have also developed a plaque reduction assay in a CD4-HeLa cell line that permits reproducible quantitative assays of drug sensitivity of these isolates. Fifty percent inhibitory (ID50) values of 18 isolates from untreated individuals ranged between 0.01muM and 0.05muM. In contrast, most isolates from patients who had received AZT for 6 months or more exhibited decreased sensitivity characterized by changes in ID50 and ID95 values, with isolates from several patients (5/15) showing 100-fold increases in ID50. The latter isolates were also insensitive to the related compound, 3'-azido, 3'deoxyuridine (AZdU), but not to the other antiretroviral compounds, dideoxycytidine (ddC), didehydrodideoxythymidine (d4T), carbovir or foscarnet (PFA). With the documentation of this potentially important phenotype and with the availability of methods to make additional isolates from other patients and to assay reliably for drug sensitivity, a number of research objectives can be pursued. We plan: Specific Aim 1: To determine the mechanism of drug resistance. Specific Aim 2: To conduct genetic characterization of the resistance mutation(s). Specific Aim 3: To determine the mechanism by which phosphatidylAZT maintains its activity against AZT resistant isolates of HIV. Specific Aim 4: To determine the virulence and in vivo susceptibility to AZT of the sensitive/resistant isolates of HIV. Specific Aim 5: To determine the frequency of emergence of AZT resistant mutants at different stages of disease. Specific Aim 6: To determine the frequency of emergence of resistant isolates during therapy with drugs other than AZT.